1. Field of the Invention
This invention relates generally to the generation and control of radio frequency (RF) transmission power, minimizing cost, and reducing the size of a portable communications device. More particularly, the invention relates to an RF amplifier that is tolerant to load variation.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and/or data communications in a handheld, telephone-like communication transceiver, a wireless personal digital assistant (PDA) or computer interface. While the different modulation and transmission schemes each have advantages and disadvantages, common goals for all of these devices is minimizing the size and cost of the handset, while maximizing the performance of the transceiver.
Minimizing the size and cost of a portable communication transceiver are typically the greatest challenges facing the designers of such devices. While many different approaches have been investigated, oftentimes the greatest opportunity for minimizing both the size and cost of the transceiver is by reducing the number of components in the transceiver. One of the devices that typically include a moderate number of components, and hence provides an opportunity for component number reduction, is the RF power amplifier. The RF power amplifier accepts the RF transmit signal and amplifies this transmit signal for transmission via the antenna associated with the transceiver.
Conventional RF power amplifiers may include multiple stages and, in some applications, include multiple amplification paths. In some configurations this multiple path arrangement is referred to as a “balanced amplifier.” Regardless of the architecture, conventional balanced power amplifiers require coupling circuitry to combine the outputs of multiple paths together. Further, because the load presented to the power amplifier changes with variations in the antenna impedance, conventional RF power amplifiers, whether those that use a single or dual path amplification scheme, generally employ one or more isolators at the output of the amplifier. Antenna impedance changes continuously depending on the location and operating mode of the transceiver. For example, the impedance of the antenna may vary between, for example, 5 and 50 ohms, depending on the location of the transceiver. Without a protective isolator or other special protective circuitry, this variation in the impedance presented to the power amplifier causes the performance of the power amplifier to vary, thus degrading overall performance of the transceiver. The isolator minimizes the impedance variation presented to the output stage of the power amplifier. Unfortunately, the isolator is a relatively large and costly component that, because of the possibility of physical contamination, is difficult to integrate onto the same structure on which the power amplifier is fabricated.
Therefore, it would be desirable to remove the isolator from the transceiver, while allowing the power amplifier to operate over a wide range of operating conditions.